OCaml Makefile Shell
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+=======================+ | cppo: cpp for OCaml | +=======================+ Introduction ============ Cppo is an equivalent of the C preprocessor for OCaml programs. It allows the definition of simple macros and file inclusion. Cppo is: - OCaml-friendly (unlike cpp) - easy to learn without consulting a manual (unlike m4 or camlp4) - reasonably fast (unlike camlmix) - simple to install and to maintain (unlike camlp4-based tools) User guide ========== Cppo is a preprocessor for programming languages that follow lexical rules compatible with OCaml. Cppo supports a number of directives. A directive is a '#' sign placed at the beginning of a line, possibly preceded by some whitespace, and followed by a valid directive name or by a number: BLANK* "#" BLANK* ("define"|"undef" |"if"|"ifdef"|"ifndef"|"else"|"elif"|"endif" |"include" |"warning"|"error" |"ext"|"endext") ... Directives can be split into multiple lines by placing a backslash \ at the end of the line to be continued. In general, any special character can used as a normal character by preceding it with backslash. 1. File inclusion ----------------- #include "hello.ml" This is how a source file "hello.ml" can be included. Relative paths are searched first in the directory of the current file and then in the search paths added on the command line using -I, if any. 2. Macros --------- This is a simple macro that doesn't take an argument ("object-like macro" in the cpp jargon): #define Ms Mississippi match state with Ms -> true | _ -> false After preprocessing by cppo, the code above becomes: match state with Mississippi -> true | _ -> false If needed, defined macros can be undefined. This is required prior to redefining a macro: #undef X An important distinction with cpp is that only previously-defined macros are accessible. Defining, undefining or redefining a macro has no effect on how previous macros will expand. Macros can take arguments ("function-like macro" in the cpp jargon). Both in the definition (#define) and in macro application the opening parenthesis must stick to the macro's identifier: #define debug(args) if !debugging then Printf.eprintf args else () debug("Testing %i" (1 + 1)) is expanded into: if !debugging then Printf.eprintf "Testing %i" (1 + 1) else () Here is a multiline macro definition. Newlines occurring between tokens must be protected by a backslash: #define repeat_until(action,condition) \ action; \ while not (condition) do \ action \ done All user-definable macros are constant. There are however two predefined variable macros: __FILE__ and __LINE__ which take the value of the position in the source file where the macro is being expanded. #define loc (Printf.sprintf "File %S, line %i" __FILE__ __LINE__) Macros can be defined on the command line as follows: # preprocessing only cppo -D 'VERSION 1.0' example.ml # preprocessing and compiling ocamlopt -c -pp "cppo -D 'VERSION 1.0'" example.ml 3. Conditionals --------------- Here is a quick reference on conditionals available in cppo. If you are not familiar with #ifdef, #ifndef, #if, #else and #elif, please refer to the corresponding section in the cpp manual. #ifndef VERSION #warning "VERSION is undefined" #define VERSION "n/a" #endif #ifndef VERSION #error "VERSION is undefined" #endif #if OCAML_MAJOR >= 3 && OCAML_MINOR >= 10 ... #endif #ifdef X ... #elif defined Y ... #else ... #endif The boolean expressions following #if and #elif may perform arithmetic operations and tests over 64-bit ints. Boolean expressions: defined ... followed by an identifier, returns true if such a macro exists true false ( ... ) ... && ... ... || ... not ... Arithmetic comparisons used in boolean expressions: ... = ... ... < ... ... > ... ... <> ... ... <= ... ... >= ... Arithmetic operators over signed 64-bit ints: ( ... ) ... + ... ... - ... ... * ... ... / ... ... mod ... ... lsl ... ... lsr ... ... asr ... ... land ... ... lor ... ... lxor ... lnot ... Macro identifiers can be used in place of ints as long as they expand to an int literal, e.g.: #define one 1 #if one + one <> 2 #error "Something's wrong." #endif 4. Source file location ----------------------- Location directives are the same as OCaml and are echoed in the output. They consist of a line number optionally followed by a file name: # 123 # 456 "source" 5. Messages ----------- Warnings and error messages can be produced by the preprocessor: #ifndef X #warning "Assuming default value for X" #define X 1 #elif X = 0 #error "X may not be null" #endif 6. Calling an external processor -------------------------------- Cppo provides a mechanism for converting sections of a file using and external program. Such a section must be placed between #ext and #endext directives. $ cat foo ABC #ext lowercase DEF #endext GHI #ext lowercase KLM NOP #endext QRS $ cppo -x lowercase:'tr "[A-Z]" "[a-z]"' foo # 1 "foo" ABC def # 5 "foo" GHI klm nop # 10 "foo" QRS In the example above, "lowercase" is the name given on the command-line to external command 'tr "[A-Z]" "[a-z]"' that reads input from stdin and writes its output to stdout. 7. Escaping ----------- The following characters can be escaped by a backslash when needed: ( ) , # In OCaml # is used for method calls. It is usually not a problem because in order to be interpreted as a preprocessor directive, it must be the first non-blank character of a line and be a known directive. If an object has a define method and you want # to appear first on a line, you would have to use \# instead: obj \#define Line directives in the usual format supported by OCaml are correctly interpreted by cppo. Comments and string literals constitute single tokens even when they span across multiple lines. Therefore newlines within string literals and comments should remain as-is (no preceding backslash) even in a macro body: #define welcome \ "********** *Welcome!* ********** " 8. Concatenation ---------------- CONCAT() is a predefined macro that takes two arguments, removes any whitespace between and around them and fuses them into a single identifier. The result of the concatenation must be a valid identifier of the form [A-Za-z_][A-Za-z0-9_]+ or [A-Za-z], or empty. For example, #define x 123 CONCAT(z, x) expands into: z123 However the following is illegal: #define x 123 CONCAT(x, z) because 123z does not form a valid identifier. CONCAT(a,b) is roughly the equivalent a##b in cpp syntax. 9. Stringification ------------------ STRINGIFY() is a predefined macro that takes one argument, removes any leading and trailing whitespace, reduces each internal whitespace sequence to a single space character and produces a valid OCaml string literal. For example, #define TRACE(f) Printf.printf ">>> %s\n" STRINGIFY(f); f TRACE(print_endline) "Hello" is expanded into: Printf.printf ">>> %s\n" "print_endline"; print_endline "Hello" STRINGIFY(x) is the equivalent of #x in cpp syntax. 10. Detailed command-line usage and options ------------------------------------------ Usage: ./cppo [OPTIONS] [FILE1 [FILE2 ...]] Options: -D DEF Equivalent of interpreting '#define DEF' before processing the input -U IDENT Equivalent of interpreting '#undef IDENT' before processing the input -I DIR Add directory DIR to the search path for included files -o FILE Output file -q Identify and preserve camlp4 quotations -s Output line directives pointing to the exact source location of each token, including those coming from the body of macro definitions. This behavior is off by default. -n Do not output any line directive other than those found in the input (overrides -s). -version Print the version of the program and exit. -x NAME:CMD_TEMPLATE Define a custom preprocessor target section starting with: #ext "NAME" and ending with: #endext NAME must be a lowercase identifier of the form [a-z][A-Za-z0-9_]* CMD_TEMPLATE is a command template supporting the following special sequences: %F file name (unescaped; beware of potential scripting attacks) %B number of the first line %E number of the last line %% a single percent sign Filename, first line number and last line number are also available from the following environment variables: CPPO_FILE, CPPO_FIRST_LINE, CPPO_LAST_LINE. The command produced is expected to read the data lines from stdin and to write its output to stdout. -help Display this list of options --help Display this list of options ------------------------------------------------------------------------------ Martin Jambon <email@example.com>